The present invention relates to a positive-working light-sensitive mixture which is especially suitable for use in the production of lithographic printing plates.
Positive-working light-sensitive mixtures on a basis of 1,2-naphthoquionone diazides, which can be used in the production of printing plates, have been known for a long time. Mixtures comprising compounds which form strong acids upon exposure and compounds which can be split by acid, for example, acetals and orthocarboxylic acid esters, have also recently been described for the same application, for example, in German Offenlegungsschriften No. 26 10 842 and No. 27 18 254. These mixtures preferably contain a binder which is insoluble in water and soluble in aqueous alkaline solutions, by preference a novolak.
Printing forms which are obtained by exposing and developing light-sensitive materials of this kind yield considerably larger print runs, when the developed plates are heated for some minutes to temperatures in the range from 200.degree. C. to 250.degree. C. In the case of light-sensitive layers containing novolaks as the binders, this heat treatment gives rise to an oleophilic deposit in the non-image areas, which consists of decomposition products of the image areas and can only be removed again by a thorough treatment with alkaline solutions.
According to published European Patent Application No. 111 274, this disadvantage can be obviated by using a halogenated polyvinylphenol as the binder. When a printing form containing a binder of this type is heated or baked respectively, there are no deposits formed in the non-image areas. However, these plates have the disadvantage that their image areas are sensitive to alkaline solutions, for example, to the developer acting over a prolonged period or to a dampening solution containing a small proportion of alkali, and they are attacked by these solutions. Usually, this will become evident as a reduction of print run.
It is known from U.S. Pat. No. 3,869,292 to use polyvinylphenol itself as the alkali-soluble binder. Compared with novolaks, polyvinylphenol has advantages for special applications.
Other alkali-soluble binders have only been disclosed as constituents of negative-working light-sensitive materials. Published European Patent Application No. 48 876, for example, describes mixtures comprising diazonium salt polycondensation products and polymers, for example polyvinyl acetals, with sulfonylurethane side groups, for use in the production of negative-working printing plates. It has been found that the alkali-soluble polymers described in this publication cannot be used in positive-working layers, for example, layers based on 1,2-quinone diazides, since they are too readily soluble in aqueous alkali, and the layers produced will therefore have an insufficient developer resistance and yield print runs which are too short.